The present invention generally relates to a golf ball, and more particularly, to a golf ball provided with improved dimple construction.
Conventionally, with respect to the pattern or configuration of dimples on a golf ball, there have been many proposed and actual techniques mainly for the purpose of improving flight performance of the golf ball.
Such conventional techniques as referred to above may be broadly divided into one technique which intends to optimize individual shapes of uniform dimples (i.e., diameter, depth, cross sectional shape, etc. of the dimple) as disclosed, for example, in Japanese Laid-Open Patent Applications Tokkaisho Nos. 60-96272, 60-163674, 58-25180, 49-52029, etc., and the other technique which defines the interval or pitch between dimples within a predetermined range as disclosed, for example, in Japanese Patent Publication Tokkosho No. 58-50744 and Japanese Laid-Open Patent Application Tokkaisho No. 53-115330, another technique which proposes a mode for arranging all the dimples at an equal pitch as shown in Japanese Laid-Open Patent Application Tokkaisho No. 57-107170, etc., and still another technique in which portions without dimples are uniformly arranged on the spherical surface of the golf ball as disclosed in Japanese Patent Publication Tokkosho No. 57-22595.
What is common to these known techniques is that they are based on the assumption that the individual dimple dimensions are the same for all. Originally, since the golf ball is a spherical body which flies in a golf game at high speeds of 20 to 80 m/sec, and also through rotation at high speeds of 2,000 to 10,000 rpm, it has been conventionally thought that the concave and convex portions or undulation on the spherical surface of the golf ball affect the force of air flow as dimensions on the average.
Meanwhile, the role of dimples in a golf ball resides in one aspect that such dimples reduce the pressure resistance by accelerating transition of a turbulent flow at the boundary layer to cause a turbulent flow separation, thereby shifting the separating point backwards as compared with a laminar flow separation in a golf ball without having any dimples, so as to decrease the separating region for the consequent reduction of pressure resistance. In addition lift is increased between the upper and lower separating points. Moreover, such role must be effectively utilized all through the range from a low speed to a high speed.
However, when dimples of the same dimensions are arranged on the surface of a golf ball as in the prior are techniques referred to above, although the maximum effect is available at the flying speed in which the dimples of that shape act most effectively, such dimples do not effectively function at other flying speed regions, thus presenting certain problems in the overall performance of the ball.
On the other hand, with respect to the relation between the surface roughness of a spherical body and the resistance force as drag thereof, many studies have been made, and there is the trend that, as the surface roughness becomes large in comparison with the resistance force in a smooth ball, the resistance force at a critical Reynolds number is increased, with the result that a reduction in the critical Reynolds number is produced. In the case of dimples for a golf ball, which is different from the roughness resulting from surface flaws, etc., the increase of the resistance force is small in the region exceeding the critical Reynolds number, but so far as the above trend is concerned, a similar tendency may also be noticed in the golf ball.
Meanwhile, the critical Reynolds number of a smooth ball is by far larger than that in the actual range of a golf ball, and is shifted towards a low speed region as the surface roughness is increased so as to be brought into the actual range of the golf ball.
Accordingly, in the golf balls, for example, if the dimple diameter is increased, the critical Reynolds number is lowered, and the resistance force in the low speed region is reduced, with an increasing trend of the resistance force in the high speed region. The trend similar to the above is also noticed when the number of dimples is increased or the depth of the dimples is increased to a certain extent. On the contrary, when the diameter and the number of the dimples are reduced or the depth of the dimples is decreased to a certain extent, the critical Reynolds number is raised, with the tendency that the resistance force in the low speed region is increased, while that in the high speed region is decreased.
Accordingly, in the available prior art no dimples which may display the maximum effects within the whole region ranging from the high speed period immediately after hitting up to peak flight, and also from peak flight to the low speed period leading to falling, thus presenting a limit to the improvement, although various studies were made into the dimple arrangement, etc. In other words, when the number or the diameter of the dimples is small, although the golf ball is allowed to fly favorably, extending over a long distance immediately after hitting, it is subjected to a so-called "hop" phenomenon which rises in the vicinity of the flight peak so as to fall at an obtuse angle, thus resulting in a loss in the flying distance at the latter half of the flight. In the case contrary to the above, the golf ball flies extending over a long distance to fall at a relatively acute angle, without the "hop" in the vicinity of flight peak, but it does not fly over a sufficient distance immediately after the flight, resulting in a loss of flying distance at the first half of the flight.
Meanwhile, together with the circumstances related to the resistance force as described above, there is a problem related to lift. More specifically, at the high speed region above a transition region, when the number or the diameter of dimples is large, or the dimple is deep to a certain extent, it is advantageous because of less influence by the wind, although disadvantageous from the aspect of the flight distance due to a small lift on the whole.
On the other hand, when the dimple arranging pattern itself is brought into question, there is a necessity for making the pattern non-directional as far as practicable, and various proposals have been made up to the present, some of which are as stated hereinbelow.
A first example which includes about 336 dimples arranged on a regular octahedron, or which includes 416 dimples as disclosed in Japanese Laid-Open Patent Application Tokkaisho No. 60-111665, a second example which includes 360 dimples arranged on a regular dodecahedron as disclosed in Japanese Patent Publication Tokkosho No. 57-22595, a third example which has 252 dimples, 432 dimples or 492 dimples arranged on an icosahedron as disclosed in Japanese Laid-Open Patent Application Tokkaisho No. 49-52029 or No. 60-234674, a fourth example which includes about 332 dimples by omitting one row or about 392 dimples by increasing one row at a seam portion of an icosahedron arrangement as disclosed in Japanese Patent Publication Tokkosho No. 58-50744, a fifth example which includes approximately 280 to 350 dimples arranged in concentric circles as disclosed in Japanese Laid-Open Patent Application Tokkaisho No. 53-115330, and a sixth example having 320 dimples disposed through an equal interval or pitch in a regular icosahedron arrangement as disclosed in Japanese Laid-Open Patent Application Tokkaisho No. 57-107170, etc.
In the above known examples, the fourth or fifth arranging pattern has a strong directivity in the arrangement of the dimples, and shows a difference in a trajectory of the golf ball according to a rotating axis upon hitting of the golf ball, thus being out of question from the viewpoint of the directivity elimination. Meanwhile, other arranging patterns may be considered favorable in the sense of non-directivity.